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Université Savoie Mont Blanc Université Joseph Fourier


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Home page > PhD fellowships > Previous Calls > Proposed PhD subjects 2013-2016 > ATLAS/top


by Yannis Karyotakis - 25 February 2013

Topics : Search for nex physics in single-top production using the ATLAS detector at the LHC
Proponents : LPSC : Arnaud Lucotte - Annick Lleres
Address : LPSC  53 rue des Martyrs 38026 Grenoble Cedex
Phone : +33 4 76 28 41 02 or 41 89
Contact Email : ;


The purpose of this PhD thesis is to search and identify phenomena beyond the Standard Model of particle physics using collisions producing top quarks in the ATLAS detector at CERN’s Large Hadron Collider (LHC) in Geneva. This search for new physics signs and the identification of the underlying theory is at the heart of the LHC project and among the most prominent questions in the field. This is even truer after the discovery of a new resonance consistent with a scalar Higgs boson claimed in 2012 by both the ATLAS and CMS collaborations.
Our approach is based on the use of the top quark, the heaviest elementary particle observed so far, as a probe to new physics sources. Several millions of top quarks events have been produced at the LHC during the last two years at 7 and 8 TeV with 25/fb of integrated luminosity. Such rate is expected to improve by an order of magnitude with the run at 13 TeV whose start is scheduled in 2015. Such high statistics allows for the first time to achieve precision measurement of the top quark rates and properties. At the same time such large datasets also enhance the sensitivity to direct searches of new signatures and phenomena that can manifest in top quark events. The extension of the sensitivity to new physics sources is driving in turn developments of theoretical efforts aimed at providing theoretical frameworks to interpret of such an enlarged panel of signatures.
Within this context, our PhD proposal is based on the combination of indirect searches from precision measurements of top quark properties and their interpretation. This implies getting familiar with the phenomenological models beyond the Standard Model, preferentially in collaboration with phenomenologists experts in the field present in the LPSC and in other IN2P3 laboratories. In particular, this topic should be focused around the characterization and the interpretation of the measured electroweak production of top quark (called single-top quark production) in terms of new physics, via the measurement of the associated cross-section and top polarization.
Single-top quark production proceeds through three modes, the s-channel, the t-channel and the associated production of a top quark and a W boson, the tW mode. The candidate is expected to take in charge the measurement of one of these processes within the top working group of ATLAS, which involves the design of a specific selection procedure and an estimate of the main background events to the signal. The combination of the measurements of these three modes measured within ATLAS is also an important aspect of this position. It implies getting familiar with selections details and systematic uncertainties affecting the measurements in view of their correlation implementation. A significant fraction of work should be devoted to understanding and adapting statistical tools used to fit simultaneously single-top cross-sections.
A significant fraction of the work should be devoted to the interpretation of these measurements, with contribution to specific tools used to interpret single-top measurements in terms of new physics. It can also be devoted to a statistical framework developed at the LPSC, which allows implementing a phenomenological approach for the measurements interpretation (via effective lagrangian) which allows modeling the impact of new interactions on the observables experimentally probed as function of parameters of relevance for the theoretical frameworks tested. This approach is aimed at providing interpretation of new physics signs in a wide range of theoretical frameworks (SUSY, Technicolor, Extra-Dimension).